1. Field of the Invention
The present invention relates to methods for making metal particles spherical and removing an oxide film from a metal, a solder paste and a soldering method. In the fields of manufacture of electronic devices and parts, recent demands for high density packaging require solder pastes and balls and soldering methods which permit fine pattern metal masks to be used and have good soldering properties.
2. Description of the Related Art
In conventional solder pastes, solder particles are changed from a non-spherical shape to a spherical shape to permit a fine pattern printing.
Manufacture of solder particles is typically an atomizing method in which a melted solder is atomized. When solders are manufactured by atomization, the resultant solder particles are usually non-spherical and have a relatively thick oxide film on the surface thereof formed during solidification of the atomized melted solder. The formation of the oxide film can be suppressed by atomization of a solder in a non-oxidizing atmosphere but this non-oxidizing atmosphere with a lower oxygen concentration necessitates a large scale apparatus and the production cost is increased. Accordingly, the present invention has a first object to provide a method for converting non-spherical solder particles having a relatively thick oxide film formed by atomization to spherical tarnish-free solder particles (spherical solder particles without an oxide film).
When a solder paste comprising spherical solder particles is formulated, the solder paste is undesirably moved or flown during or after metal mask printing. To solve this problem, irregular shaped particles may be combined with the spherical solder particles in the solder paste. Nevertheless, changing fine spherical tarnish-free solder particles, which can be manufactured by a process-provided by the present invention, to irregular shaped particles by machining as a practical basis is difficult. On the other hand, if irregular shaped solder particles manufactured by atomization are used, the irregular shaped solder particles have a relatively thick oxide film, as mentioned above, and the shelf life of the solder paste becomes remarkably shortened.
Therefore, the second object of the present invention is to provide a method for converting irregular shaped solder particles having a relatively thick oxide film formed by atomization to tarnish-free solder particles, the solder particles retaining the original or irregular shape.
A solder paste is a mixture of solder particles and a flux vehicle. The flux vehicle has a function of permitting the solder paste to be supplied by printing, dispensing, etc. Specifically, the flux vehicle usually comprises rosin, an activator or fluxing agent, an organic solvent and hardened caster oil. The activator is added to decompose and remove an oxide film existing on a portion where a soldering is to be conducted and is typically amine hydrohalogenide such as diethylamine hydrochloride, which must be removed by a cleaning solvent after the soldering. The rosin is added to shield the portion to be soldered from the exterior and maintain a non-oxidizing atmosphere. The organic solvent is a vehicle to dissolve and supply the rosin and activator to the portion to be soldered and is typically diethylene glycol monoethyl ether (b.p. 201.9.degree. C. ). Most organic solvent is evaporated during a preheating step of soldering and may not exist when soldering is being conducted.
If an activator or fluxing agent is not contained in the flux vehicle, the thick oxide film of solder particles formed during manufacture of the particles cannot be decomposed during soldering so that the solder particles remain on the portion to be soldered as large solder balls and good soldering cannot be conducted. Accordingly, undecomposed activator remains in a remaining flux vehicle after the completion of the soldering and, if it is not removed by cleaning, it corrodes the soldered portion and lowers the reliability of the device. Nevertheless, in recent bare chip packaging, i.e., where LSI chips are soldered directly onto a circuit board, soldering portions have a size or diameter of about 100 .mu.m and a gap between the LSI chips and circuit board is only about 70 .mu.m, which makes the flux cleaning after soldering difficult.
Accordingly, the third and fourth objects of the present invention are to provide a solder paste and soldering method in which flux cleaning after soldering is not necessary.